IPCC deputy chair Rachel Cerfontyne said: "As part of our investigation, the IPCC has uncovered evidence that the chief constable may have breached the Data Protection Act by sending emails concerning police business, some of which contained personal data, to individuals unconnected to the force.

"On July 8 I formally brought this matter to the attention of the police and crime commissioner for Avon and Somerset who referred Mr Gargan's conduct to the IPCC the following day. Mr Gargan has now been served with a notice informing him that this aspect of his conduct is subject to a criminal investigation."

The police watchdog has so far interviewed more than 50 members of staff from the force and sifted through computer and telephone data as part of its probe.

Avon and Somerset Police and Crime Commissioner Sue Mountstevens said he had been made aware of new evidence in the IPCC's ongoing investigation into the conduct of Mr Gargan.

"I have referred the matter to the IPCC to investigate and the chief constable remains suspended," she said.